Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings and Other Financing Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments in Next Twelve Months	$ 151
Long-term Debt, Maturities, Repayments of Principal in Year Four	19
Long-term Debt, Maturities, Repayments of Principal in Year Five	1
Deferred Finance Costs, Noncurrent, Net	7	$ 5
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met	$ 53
Minimum calendar year average equity to total capitalization ratio authorized by state commission	52.50%	52.50%
Calendar year average equity to total capitalization ratio	53.10%
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 19
Expected equity to total capitalization ratio for the next fiscal year	51.50%
First Mortgage Bonds | Series Due Dec. 1, 2047
Debt Instrument [Line Items]
Interest Rate, Stated Percentage	3.75%
Maturity Date	Dec. 01, 2047
NSP-Wisconsin | First Mortgage Bonds | Series Due Dec. 1, 2047
Debt Instrument [Line Items]
Face Amount	$ 100
Interest Rate, Stated Percentage	3.75%
Maturity Date	Dec. 01, 2047